Currently, thin wires (bonding wires) having a wire diameter of 20 to 50 μm or so are popularly used as bonding wires for connecting an electrode on a semiconductor element and an external terminal together. A thermal compressive bonding technique with the aid of ultrasound is generally applied to bond bonding wires, and a general-purpose bonding device, and a capillary jig which allows a bonding wire to pass through the interior thereof for connection are used. A leading end of a bonding wire is heated and melted by arc heat inputting, a ball is formed by surface tension, and then the ball is compressively bonded on an electrode of a semiconductor element heated within a range from 150 to 300° C. beforehand. Thereafter, the bonding wire is directly bonded to an external lead by ultrasound compressive bonding.
Recently, techniques related to the structure, material and connection for the semiconductor packaging technologies are rapidly diversified, and for example, in a packaging structure technology, in addition to currently-used QFP (Quad Flat Packaging) using a lead frame, new configurations, such as BGA (Ball Grid Array) using a substrate, a polyimide tape or the like and CSP (Chip Scale Packaging) are practically used, and a bonding wire which has improved loop characteristic, bonding property, mass productivity, usability and the like becomes requisite. Among those improved bonding wire connecting techniques, the wedge-to-wedge bonding technique suitable for fine pitch connection, other than the currently mainstream ball/wedge bonding techniques, requires the fine wire bondability to be improved as the wire is directly bonded at two locations.
Materials to be bonded with the bonding wire have also become diversified. In addition to the conventional Al alloy, copper suitable for a finer wiring has been practically used as a wiring and electrode material on silicon substrates. Ag-plating and Pd-plating are applied onto an upper portion of a lead frame, while copper wirings are applied onto upper portions of a resin substrate, a tape or the like, onto which a film of a noble-metal element such as gold or its alloy is often applied. Depending on a variety of these bonding target materials, a bonding wire is required to be improved in bondability and reliability.
So far, 4N-group gold having a high purity (purity>99.99 mass %) has been mainly used as a material of a bonding wire. Gold is, however, expensive and hence a bonding wire made of another metal material which is less expensive has been desired.
According to the requirements of the wire bonding techniques, it is important to form a ball with a good sphericity at the time of forming the ball and to obtain a sufficient bonding strength in a bonded portion between the ball and an electrode. Further, to cope with lowering of a bonding temperature, thinning of a bonding wire, etc., a bonding strength, a tensile strength and the like are requisite at a part where a bonding wire is subjected to wedge bonding to a wiring on a circuit wiring board.
In the resin encapsulation process of injecting high-viscosity thermosetting epoxy resin at high speeds, there occurs a problem that a bonding wire is deformed to come in contact with the adjacent wires, and besides the wire deformation is required to be restrained as much as possible, in a resin encapsulation process under the situations where pitches become finer, wires become longer and thinner. Although such deformation can be controlled to some extent by an increase in wire strength, this resin encapsulation process still involves difficulties in putting it for practical application, unless some problems that loop control becomes difficult and the strength in bonding decreases, etc. are solved.
Besides, long-term reliability is also important at the time of actual use of a semiconductor element on which a bonding wire is connected and mounted. Particularly for a semiconductor element or the like mounted on an automobile, high reliability under such severe environments as a high temperature, high humidity, heat cycle or the like is required in order to ensure strict safety. Even in such unprecedented severe environments, high reliability must be maintained without deteriorating a bonded portion where the bonding wire has been connected.
As wire characteristics for satisfying the above requirements, it is desired that overall characteristic features thereof be satisfied, such as easy loop control in a bonding process, improved bondability to an electrode and a lead wire, restrained excessive wire deformation in a resin encapsulation process subsequent to the bonding process, and furthermore, long-term reliability in a connected portion as well as stability at a bonded portion under severe environments.
A bonding wire made of copper has been developed in order to achieve low material cost, excellent electric conductivity, enhanced ball bonding and wedge bonding properties, as is disclosed in Japanese unexamined patent application publication No. S61-99645, etc. According to the copper bonding wire, however, there occur problems that the oxidization of the wire surface reduces bonding strength and the wire surface is prone to generate corrosion or the like at the time of resin encapsulation. These problems are partially responsible for the lack of progress in practical application of the copper bonding wire.
In the copper-based bonding wire, when forming a ball by melting a wire tip, the bonding process is performed with a gas sprayed onto the wire tip in order to inhibit oxidation. Currently, a nitrogen gas containing 5 vol % hydrogen is generally employed as an atmospheric gas used in forming a ball of the copper-based bonding wire. In Japanese unexamined patent application publication No. S63-24660, it is disclosed that when a copper wire is bonded to a copper lead frame or a copper alloy lead frame, the bonding process is performed in the atmosphere containing 5 vol % H2+N2. Also, in “Copper Ball Bonding for Fine Pitch, High I/O Devices”: P. Devlin, Lee Levine, 38th International Symposium on Microelectronics (2005), P. 320-324 is reported that in forming a ball of the copper bonding wire, the 5 vol % H2+N2 gas can prevent a ball surface from being oxidized and therefore, the 5 vol % H2+N2 gas is more desirable than a N2 gas. Today, the 5 vol % H2+N2 gas has been standardized as a gas used in employing the copper-based bonding wire.
As a technique of suppressing any oxidization of a surface of a copper bonding wire, Japanese unexamined patent application publication No. S62-97360 discloses a bonding wire in which copper is covered with a noble metal or a corrosion-resistant metal, such as gold, silver, platinum, palladium, nickel, cobalt, chrome, titanium, and the like. Moreover, from the standpoint of a ball formability and suppression of deterioration of a plating solution, Japanese unexamined patent application publication No. 2004-64033 (“JP '033”) 4 discloses a bonding wire so structured as to have a core member mainly composed of copper, a dissimilar metal layer formed on the core member and made of a metal other than copper, and a coating layer formed on the dissimilar metal layer and made of an oxidization-resistant metal having a higher melting point than copper. Japanese unexamined patent application publication No. 2007-12776 discloses a bonding wire comprising a core member mainly composed of copper, and an outer skin layer which contains a metal, having either one of or both of a constituent and a texture different from the core member, and copper, and which is a thin film having a thickness of 0.001 to 0.02 μm.
The conventional copper bonding wire with a monolayer structure (i.e., a non-coated copper bonding wire which, in some cases, is formed with a thin and naturally oxidized film layer on its wire surface. Hereinafter called a monolayer copper wire), has practical problems that the wire surface is easily oxidized, decrease in bonding strength is prone to occur, etc. Thus, as a means for preventing the oxidation of the surface of the copper bonding wire, the wire surface may be coated with a noble metal or oxidation-resistance metal.
Study by the present inventors in view of the needs for density growth, miniaturization, thinning or the like in the semiconductor packaging technology has revealed that the conventional multilayer copper wires with its surface coated with a metal different from copper (herein, a non-coated copper wire is called a monolayer copper wire, while a copper wire coated with one layer is called a multilayer wire, and thus, hereinafter called a conventional multilayer copper wire), have lots of hereinbelow-mentioned practical problems that remain unsolved.
When a ball is formed on a tip of the conventional multilayer copper wire, a flat ball formed with a decreased degree of sphericity, an unmelted part of the wire remaining inside the ball, and the generation of bubbles becomes problems. If such irregular balls are bonded on electrodes, reduction in bonding strength, chip damage or the like are caused.
When performing a complicated loop control using the conventional multilayer copper wire, a coated layer and the copper may peel from each other between their interfaces, and therefore, there occur concerns that a loop shape may become unstable, and adjacent wires may become electrically short-circuited in the case of fine-pitch bonding.
When forming a ball using the conventional multilayer copper wire, it is of a practical concern that a defective shape of a ball bonded portion and a reduction in bonding strength are more likely to take place than when using a monolayer copper wire or a currently mainly used gold bonding wire. To give specific failure examples, formation of a flat ball with a decreased degree of sphericity, misalignment of a ball formed obliquely relative to a wire, part of the wire remaining inside the ball, and the formation of bubbles (blow holes) sometimes become problems. If such irregular balls are bonded onto an electrode, there will occur a misalignment deformation where the ball is misaligned with the center of the wire and deformed, an elliptical deformation where the ball is deviated from sphericity, and a petal-like deformation, etc., thus leading to protrusion of the bonded portion from an electrode surface, reduction in bonding strength, chip damage, failure in production management or the like. These initial bonding failures may cause the degradation of long-term reliability, as described above.
JP '033 discloses that an outer skin layer may be formed to 0.001 to 0.02 μm thickness, as a technique for solving failures associated with the ball bonding of the conventional multilayer copper wire. The outer skin layer referred to here includes a concentration gradient region as well, and an interface between the outer skin layer and a core material, is described as having a metal M concentration of 10 mol % or more. Through the study by the inventors of the present invention, it has been observed that the above-mentioned problems concerning the ball bonded portion are partially improved by thinning the outer skin layer, yet it has been verified that such improvement does not necessarily suffice when used under new environments such as an application to a semiconductor or the like mounted on an automobile, but in fact, the thinner the outer skin layer is, the more frequently a flat ball rather occurs. Besides, it has been verified that thinning a layer makes an improvement in wedge bonding insufficient, causing a hereinafter-described problem concerning long-term reliability.
For the sake of the evaluation under new severe environments, specifically, the following test is being performed. That is, in the reliability test of a semiconductor connected with a monolayer copper wire, a temperature cycle test (TCT test) is performed, showing a wire is fractured in the vicinity of a wedge-bonded portion, and the occurrence frequency of such wire fracture is higher than that in a gold bonding wire, which is now becoming a problem. In a solder reflow process as well, there is the concern about the failures that a bonded portion of a copper-based wire is similarly fractured. This is also a type of wire fracture caused by thermal fatigue. A Pb free solder that has been rapidly put into a practical use for the sake of the environmental countermeasure is higher in a melting point than the conventional tin-lead solder, and therefore, thermal strain due to the use of Pb free solder is becoming a problem. The wire fracture is attributable to a failure caused by differences in thermal expansion among constituent members of a semiconductor such as encapsulating resin, a lead frame, a silicon chip or the like. In order to cope with an increased calorific value at the time of an operation of a semiconductor, an increase in temperature of the usage environment and increased variations in temperature of the same, it becomes important to reduce wire fracture in the TCT test in the case of the copper-based bonding wire.
The present inventors have verified that the frequency of failure occurrence under the TCT test becomes slightly decreases in the conventional multilayer copper wire as compared to in a monolayer copper wire, but it is still inferior to a gold bonding wire. For example, if the outer skin layer described above is as thin as 0.001 to 0.02 μm in the conventional multilayer copper wire, the improvement effect exhibited in the TCT test was insufficient.
As surface oxidization proceeds in a monolayer copper wire, it becomes a problem in use that its storage life is short in the atmosphere. The conventional gold bonding wire can be stored for about one month before or during use. As for the monolayer copper wire, however, storing it only for several days in the atmosphere will cause a problem that the wedge bondability is reduced and a ball shape becomes unstable, which become factors causing deterioration in the workability of the copper-based bonding wire.
According to the conventional multilayer copper wire, the effect of retarding the oxidizing process can be more expected than done by the monolayer copper wire. This effect, however, significantly varies depending on the composition, structure and thickness of an outer layer of a wire or a vicinity of a wire surface. Therefore, it is important that the structure of the conventional multilayer copper wire be optimized. To ensure the equivalent workability to a gold bonding wire, wedge bondability and loop shape, etc. need to be ensured to be degradation-free, even after storage in the atmosphere for about two months. This means that several ten times the life duration must be ensured as compared to the storage life of the monolayer copper wire, leading to considerably strict conditions required for a material whose main constituent is copper.
Among the problems associated with oxidization, inhibition of oxidation at the time of forming a ball is also an important subject of a copper wire. For the conventional monolayer copper wire, a 5 vol % H2+N2 gas is popularly employed as a standard ball-forming gas. When the 5 vol % H2+N2 gas is employed, however, the cost for providing dedicated piping arises for the sake of providing the gas in a factory, and the running cost of this mixed gas is also expensive. When comparing total costs including the manufacturing cost, the cost advantage sometimes becomes smaller as compared to that of a gold bonding wire even if copper is used for a wire material. The gas cost is one of the factors for the copper-based bonding wire not to prevail. Further, safety management becomes strict due to as much as 5 vol % content of hydrogen, leading to a concern about a reduction in workability.
If a gas for forming a ball is comprised of N2 only, a cost reducing effect is considerably enhanced and an obstacle to the safety management is reduced, and thus it has a number of advantages to a user. When using the conventional monolayer copper wire in mass production, however, a pure N2 gas has not come into practical use because of the judgment that it is difficult to use in that case. Similarly, when using the conventional multilayer copper wire, totally stable productivity is easier to ensure by using the 5 vol % H2+N2 gas, and using the pure N2 gas has led to some problems such as the occurrence of the above-mentioned misaligned ball, unstable ball size, etc. If there can be produced the conventional multilayer copper wire that is capable of providing high productivity and high reliability even when using the pure N2 gas, the obstacle that hinders the spread of copper-based bonding wire is made small, and thus its practical application can be expected to be accelerated.
Therefore, it is an object of the present invention to provide a semiconductor device bonding wire mainly composed of copper, aimed at reducing the occurrence of failures in thermal cycle tests, in addition to the conventional fundamental performance, by solving the problems associated with the conventional techniques described above.